Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...

I have no experience with home-brewed etalons, but Baader says on their webpage that their filter glass is "polished plane to a quarter wavelength on both sides", which makes me a bit sceptical if two Baader filters give a working etalon (allthough I am a big fan of tinkering with optical components and the idea of home-brewing an etalon is, of course, enticing...).

Hello
I'm not very expert, but two years ago I also thought about building an etalon in my home.
I had two optical windows calibrated with correction on all sides of lambda/20. I thought about aluminising one side of every window with 95-99% reflection.
I did not have problem doing this, I asked an optical company, but I also have an aluminizing bell in my institute. The high reflection is necessary because the "finess" is greater the larger the reflection of the two inner sides of the etalon.

The biggest problem came out when I verified that a lambda/20 correction is not enough to get an etalon. At least lambda/100 is required.
I have confirmed this by using this software

With lambda/20 the curve is almost flat, with almost zero "finesse" ...
If it does not work with lambda/20, it will work worse with two commercial filters.

This commercial filters are not better than lambda/4 .. A few months ago I did a test of about twenty quality filters , 2" and 1 1/4"(Baader, Schneider, B+W, Hoya and kenko) and I verified that they are almost all lambda/2. Only one Kenko was lambda/4 (casually).

This is the state-of-the-art value of the optical correction of good commercial filters (in the past I had tested some Chinese economic filters and the quality was much lower, about 2-4 lambda. 6 Paolarized filters of 67 mm marked Panagor had a shameful quality and the thing was strange because since panagor is not a bad brand .... The poor quality was visible with the eyes looking at the view through them. In practice, they were made with glass of windows !!!). Polarized filters have much lower quality of lambda/2 (Baader included, at least in my two Baader filters).

This does not mean anything because this correction is sufficient in the "normal" observation, also because normally the optical beam does not cross the whole filter (when the filter is inside the optical path), but only a part and, for this part, the correction is actually lamda/4 (except for polarizing filters).

So ... unfortunately it is not possible to do an etalon in own home ...

"If an airspaced etalon is just two pieces of partially reflective glass contacted between mica spacers..."

Not quite Apollo!

Polishing to lambda/100 flatness across 25 mm is not too difficult using relatively standard production techniques, achieving this over the full area of a larger filter becomes exponentially more difficult.

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Additional very difficult to achieve and important etalon functions in addition to plate flatness is plate parallelism and gap spacing (thickness) stability. For the air spaced etalons from Coronado, Lunt, and SolarScope, the spacers have to be of the precisely same thickness in order to keep the plates parallel. A thin 2 inch diameter substrate is therefore polished to the appropriate thickness to get the CWL close to what it should be but allow for tilting or pressure tuning, and small adjacent pieces are broken off to be used for a particular etalon to ensure the highest thickness uniformity:

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Generally these spacers are made of fused quartz or a similar low coefficient of thermal expansion material - mica would be completely unsuitable, as it expands and contracts with temperature changes too much to maintain the proper gap through a range of normally occurring environmental temperatures. Mica is used for solid gap etalons (DaysStar & Solar Spectrum) due to it's ability to be precisely mechanically cleaved versus polished, and the fact that it has a relatively high CTE makes it only usable at much above ambient temperatures - thus the need for heating. This high CTE also allows for the CWL to be more easily adjusted by varying the temperature of the etalon, which changes the gap length.

Finally, the reflective coatings used have to be of very high quality, and of precisely uniform thickness, and also specialized not to deform the plates or mica after application, which requires specialized coating techniques and technologies not utilized in everyday filter coating and manufacturing...

All these things add up to make a narrow band solar etalon a very different beast from the typical astronomical filter, and why they are so costly to produce and expensive to buy.

There are only 5 companies in the world making such dedicated solar filters commercially. Custom etalons made by other etalon manufactures are made one-off and therefore much more expensive. A typical 150 mm etalon for a professional observatory could easily cost in excess of $50,000 - $100,000, but of course would have a very high quality, and probably precisely peizo electrically tuned, etc:

I and you belong to the same category of people who like to experiment and who are sometimes optimistic about the feasibility of a project. It is normal that sometimes the project can fail. Not necessarily a failed project is a failure. On the contrary, a failed project is useful to face new projects with a better awareness. This is my philosophy.

How it proceeds your double stack project with beam splitter?
Now I am writing you some my considerations in the relative thread.

I and you belong to the same category of people who like to experiment and who are sometimes optimistic about the feasibility of a project. It is normal that sometimes the project can fail. Not necessarily a failed project is a failure. On the contrary, a failed project is useful to face new projects with a better awareness. This is my philosophy.